The present invention concerns laser machining apparatus and in particular apparatus that homogenizes laser illumination on a mask that is used in a projection imaging system for drilling holes in materials with multiple laser pulses.
Many types of laser drilling application project an image of a mask pattern onto the work piece by laser beam illumination. One such system is shown in U.S. Pat. No. 6,040,552 to Jain et al. entitled HIGH-SPEED DRILLING SYSTEM FOR MICRO-VIA PATTERN FORMATION AND RESULTING STRUCTURE. In this system, a laser beam illuminates a planar mask containing a pattern of open and opaque areas. The pattern of open areas (referred to from this point on as the mask) is imaged by a lens system on to the work piece surface. The pattern that is defined by the open areas of the mask is then drilled in the work piece. If the linear magnification of the imaging lens is M, the pattern on the work piece is magnified in a linear dimension by a factor of M.
The laser beam has a finite transverse size, characterized by its beam diameter. For most lasers, the transverse laser light does not have a uniform intensity. Typically, the intensity of the laser beam can be modeled as a two-dimensional Gaussian distribution, with the highest intensity at the center of the beam and lower intensities toward the edge of the beam. For laser drilling with projection imaging, a uniform laser intensity distribution at the work piece is desired so that the drilled holes are uniform. To insure uniform illumination of the mask, the mask is either much smaller than the laser beam, or, as in the above-referenced patent, the beam is transformed to a more uniform transverse intensity distribution before illuminating the mask. The process of transforming the laser beam is known as beam homogenization or beam shaping. Typical beam homogenization systems, such as that used in the above-referenced patent employ optics that change the light distribution of the laser beam.
The present invention is embodied in a method and apparatus to homogenize a laser beam in applications where multiple pulses are used for drilling in a projection imaging arrangement. As used herein, the term xe2x80x9cmultiple pulse drillingxe2x80x9d means that the image of the mask remains relatively stationary on the work piece surface over the firing of a plurality of laser pulses. According to the subject invention, a mechanical device scan the beam over a mask to illuminate a larger area of the mask than the beam size on the mask. Any non-uniform beam illumination is averaged out during the multiple pulse drilling process.
According to one aspect of the invention, the beam of laser light pulses has a central portion and a peripheral portion and the beam homogenizing apparatus moves the beam of laser light pulses in a pattern on the work piece such that, over a plurality of laser pulse intervals, each aperture is exposed to the central portion and to the peripheral portion of the laser beam.
According to another aspect of the invention, the beam homogenizing apparatus is a mirror that reflects the beam of laser light pulses onto the mask. The mirror has a planar reflecting surface mounted on the shaft of a motor which rotates around an axis and the mirror is mounted to the motor shaft such that the plane of the mirror is at an angle to a plane that is perpendicular to the axis of rotation.
According to another aspect of the invention, the beam homogenizing apparatus includes two galvo mirrors. The first galvo mirror is configured to reflect the laser beam on to the second galvo mirror and the second galvo mirror is configured to reflect the laser beam onto the mask. Each of the first and second galvo mirrors rotates about a rotational axis in a plane that is parallel to the plane of its mirror.